HVDC/MVDC grids have gained increasing attention as an alternative to ac system for long-distance power transmission and distribution as well as renewable energy collection applications. The solid-state transformer (SST) plays an essential role for flexible dc power exchange and voltage regulation in HVDC/MVDC grids. The isolated modular multilevel dc/dc converter (IM2DC) gains increasing attentions recently as a promising topology for SST application. As the single-phase topology shown in FIG. 1 and the three-phase topology shown in FIG. 2, IM2DC comprises two modular multilevel converter (MMC) connected through a medium-frequency transformer, which provides the solution to dc/dc conversion with low-rating devices, superior fault performance and lower cost in HVDC/MVDC systems.
Various modulation methodologies have been proposed in IM2DC with distinct advantages as shown in FIG. 3 using single-phase topology modulation as an example. The phase-shifted sinusoidal modulation, which is widely applied in traditional MMCs with high quality ac output, can be used in IM2DC directly. Compared to the sinusoidal modulation, the two-level (2L) modulation generates a square wave ac-link voltage, which leads to a higher efficient power transfer capability. Recently a quasi-two-level (Q2L) modulation has been proposed for not only to achieve lower dv/dt but soft-switching capability as well. In addition, triangular modulation is discussed as well which exhibits lower harmonic components than 2L modulation and simpler implementation than sinusoidal modulation.
On the other hand, the IM2DC arm voltage and current waveforms varies with the same transmission power via different modulations, which affects both total device rating (TDR) and passive component size. Large cell capacitors are needed for sinusoidal modulation. Meanwhile, triangular modulation results in large arm RMS current leading to a higher TDR and conduction loss. With 2L and Q2L modulation, smaller capacitors and devices are applicable, however, the dc current ripples of dc inductor may be larger than those who use multi-cell interleaved modulation method.
A need, therefore, exists for a IM2DC modulation method that allows for a reduction in the size of passive components without a TDR penalty and that insures efficient power transfer.